This invention relates to reactor pressure vessels (RPVs) in pressurized water nuclear reactors (PWRs). It relates particularly to RPVs in which a small fraction of inlet coolant water is diverted from the main coolant in order to cool the RPV heads.
A PWR generally includes a closed loop of pressurized coolant water to transfer heat energy from fuel assemblies in the core region of a RPV to a secondary water system employed to generate steam. The closed loop operates at pressures of up to about 2250 psi or more and at temperatures of up to about 650° F. or more. The coolant water may be heated about 60° F. (e.g., from about 550° F. to about 610° F.) in a RPV and then cooled an equivalent amount by the secondary water system.
After decades of operation at high temperature and pressure, the wetted metal surfaces of RPVs (which generally are fabricated of stainless steel and nickel base alloys) contacted by the circulating coolant water are experiencing stress corrosion cracking. One well recognized method of reducing the susceptibility of metals to stress corrosion is to reduce the temperature of the wetted metal surface. Tests have shown that crack initiation times can be reduced significantly by reducing temperatures of the RPV heads by just 10° F.
Accordingly, and in addition to other RPV thermal-hydraulic modifications, RPV components have been redesigned or modified to divert relatively cool inlet coolant water away from the main coolant water path and toward the RPV heads in order to cool the RPV heads. Thus, RPVs may have coolant water flow holes machined in the flanges of core barrel assemblies and upper support assemblies to provide a flow path for the diverted inlet coolant water. See, e.g., U.S. Pat. Nos. 5,325,407 and 4,786,461.
The inventors have found that, although the flow patterns of U.S. Pat. Nos. 5,325,407 and 4,786,461 will provide the desired cooling effects, the diverted coolant flow may not provide the expected flow of coolant water into the RPV heads in all circumstances. Specifically, it has been found that the flow of coolant water through the holes in the upper support plates can induce coolant water in the RPV heads above the upper support assemblies to leak back into the space between the flanges and dilute the diverted coolant water somewhat like a jet pump aspirating surrounding fluid. In one test, the loss factor (“k”) of Bernoulli's equationPressure Differential=k(Velocity)2/2(gravitational constant)was determined to be 1.6 where a loss factor of 1.1 was employed in the design of the holes.
Thus, the quantity of diverted flow of coolant water into the RPV head and its temperature may not be sufficient to cool the head in accordance with the design.